The human body contains many joints that permit articulation of varying degrees between bones. Those that permit free articulation are referred to as diathroses. Examples include the hip, knee, elbow and shoulder. A variety of connective tissues are associated with the diathroses joints, including intra-articular cartilages that provide cushioning and smooth sliding surfaces, ligaments that provide flexible connections between bones and tendons that slide over joints and connect the muscles to provide motion. When connective tissues are compromised, joint pain and loss of function can result.
One example of compromised connective tissue is osteoarthritis of the knee or knee OA. The knee joint is formed by the articulation of the femur, patella, and tibia (FIGS. 1A and 1B). Like other freely articulating joints, the knee joint is enclosed by a fibrous joint capsule, lined by a synovial membrane. The inferior surface of the patella articulates with the femoral surface forming the patellofemoral joint. The distal end of the femur has two curved articular surfaces called the medial and lateral condyles. These surfaces articulate with the medial and lateral tibial condyles, forming the tibiofemoral joint, which flexes and extends the knee. Two fibrocartilagenous discs (i.e., menisci) lie between the tibial and femoral condyles to compensate for the incongruence of the articulating bones. Because the distal end of the femur is curved and asymmetric in shape, the knee joint not only flexes and extends like a hinge, but it also slides and rotates during flexion, resulting in a complex motion for the joint.
Knee OA is one of the most common causes of disability in the United States. OA is sometimes referred to as degenerative, or wear and tear, arthritis. OA is characterized by the breakdown of the articular cartilage within the joint. Over time, the cartilage may wear away entirely, resulting in bone-on-bone contact. Since bones, unlike cartilage, have many nerve cells, direct bone contact can be very painful to the OA sufferer. In addition to the pain and swelling, the OA sufferer can experience a progressive loss of mobility at the knee joint. This is due to loss of the joint space, where the articular cartilage has completely worn away.